A total of 65 elite swimmers from seven countries (Australia, Brazil, China, Spain, Netherlands, Slovenia, and Tunis) are taking part in the ALTITUDE Project, the largest altitude training research project ever. Initial testing was carried out in Barcelona (Sabadell) and Madrid. Four altitude and sea-level training camps were set up in Sierra Nevada (CAR), Sabadell (Barcelona), and Madrid (CSD). Scientists from Spain, Finland, Germany, Slovenia, Scotland, USA, and China are collaborating in fieldwork and others will join in data analyses and publication. The project will cover different topics including performance, physiological capacities, hematological response, immune system status, pro-oxidant/antioxidant balance, body composition, and cardiovascular adaptation.